Underdrain filtration system with stamped perforations

ABSTRACT

An underdrain filtration system has improved perforation structure, the perforations being positioned to admit the filtrate into a plenum defined by elongated underdrain rib structures of the system. Instead of drilled or laser-cut holes or slits, the slits are formed by stamping discs from the stainless steel plate, such that the discs extend parallel to the surrounding flat area but are raised just sufficiently to define a plurality of slits under each disc, preferably two to four slits, each being an arc of slightly less than 180°, 120° or 90°, respectively. A pair of metal bridges remain to integrally connect the raised disc and the surrounding flat field. Consistent perforation width is achieved, at, for example, 0.010″ width varying by less than 0.001″ in either direction, improving consistency of pressure drop. In addition, the raised discs during backflushing of the underdrain create an outwardly diverging water flow pattern which is beneficial in clearing trapped particles from the filter medium in the vicinity of the underdrain perforations.

BACKGROUND OF THE INVENTION

[0001] This invention concerns underdrain filtration systems for waterpurification or sewage treatment systems which employ a filtering mediumof stacked granular material retained above a tank floor by perforatedmetal underdrain structures. More particularly, the invention isconcerned with an improvement whereby the perforations in the metalunderdrain structures supporting the granular material are formed in anefficient manner so as to establish a high rate of consistency among theopening sizes, as well as promoting a better fluid flow pattern duringbackflushing.

[0002] An example of an underdrain filtration system is shown inapplication Ser. No. 09/170,870, U.S. Pat. No. 6,090,284. As pointed outin that application and patent, such granular filtration systemstypically have corrugated or undulating-shaped perforated underdrainstructures, and the systems are equipped to backwash the granularmaterial covering the underdrain units by water, and usually by air froman adjacent source. The assignee's U.S. Pat. No. 4,331,542 alsodescribes an underdrain structure for use with such gravity-fed granularfiltration systems, fitted with water backflushing and air scouring.

[0003] As explained in those patents, the subject high rate gravityfilters usually have filter beds with gravel of varying sizes. Liquidfrom the tank or vessel flows downwardly by gravity through the filtermedium and exits the filtration tank through openings in the underdrainstructures that support the granular filtration medium. Typically thecoarsest gravel of the medium is at the bottom, against the underdrainstructure, with the gravel decreasing in size up to the finest gravel orsand, located at the top of the filter bed. Some filter beds are allsand, and the bed may be covered with finely granulated anthracite coalor other types of filter material.

[0004] Backwashing and air scouring are important aspects of these typesof gravity filters, for cleaning the filters against clogging. Water ora combination of air (or gas) and water is passed up through the filterbed, in the opposite direction of normal filtration. Air may be from atube adjacent to the underdrain structures. Uniform distribution of thebackwashing fluids is a goal of most underdrain structures. Thisincludes the systems described in the above-referenced patents, as wellas some of the underdrain systems of the following patents: U.S. Pat.Nos. 4,659,462, 4,707,257, 5,015,383, 5,019,259, 5,156,738, 5,160,614,5,269,920, 5,332,497, 5,462,664, 5,489,388, 5,512,174, 5,639,384.

[0005] Application Ser. No. 09/170,870, U.S. Pat. No. 6,090,284, showsan embodiment of an underdrain structure wherein the water-passingperforations are in the form of narrow slits, small enough to support abed of sand without allowing the sand to pass through the openings. Slitopenings were made by laser cutting, as opposed to an earlier practiceof forming the openings as {fraction (3/16)}″ diameter holes, over whichgravel was placed.

[0006] The consistency of the opening sizes is important for properwater distribution in backflushing and balancing of pressure drop acrossthe entire underdrain structure. Laser cutting is fairly expensive andhas typically not been capable of providing a highly consistent range ofopening width. The openings formed by laser cuts in the experience ofthe applicant ranged from about 0.008″ to 0.012″, a variation of 0.002″in both directions from the target size of 0.010″. The resultingdifferences in pressure drop tended to cause inefficiency in the actionof the backwashing water in cleaning the surrounding filter medium.Moreover, even if these slots could be formed perfectly at the targetwidth, the slots would not provide an optimum pattern of pressurizedbackwash water flow for filter cleaning, as does the system of theinvention described below.

SUMMARY OF THE INVENTION

[0007] The invention is an improvement in the fabrication andperformance of the filter media-retaining underdrain. Instead of roundholes or laser-cut slots, the invention utilizes a machine punch processto produce fine openings in a metal plate of specific thickness gauge,resulting in openings of a very consistent width. In one embodiment theopenings are 0.010″ in width and sufficiently small and consistent todirectly retain a filter medium (sand) ranging from 0.3 mm to 0.5 mm.The punched plate preferably is then folded into a trapezoidal shapesuch as shown in the above-referenced U.S. Pat. Nos. 4,331,542 and6,090,284. The trapezoidal underdrain structure defines a conduit forcarrying filtered water or wash water in a municipal water treatmentfilter or tertiary treatment in a municipal waste water treatmentfilter.

[0008] The stampings preferably are circular in shape, although theycould be other curved shapes or polygonal, and produce a plurality ofslots which lie generally in a circuit defined by the space between araised disc and the surrounding flat area of the underdrain material. Inone preferred embodiment the raised discs are circular, with two to fourbridges between ends of adjacent slits, these metal bridges being leftremaining from the stamping of the discs. Two such bridges between twoarcuate slots are generally sufficient, but three bridges will providemore stability of the aperture sizes in the event of rough handling orother forces experienced during transportation, installation and use.

[0009] A very highly consistent slit width can be formed in this way,varying less than 0.001″ in either direction, and thereby improvingconsistency of pressure drop throughout the underdrain. As a result ofthe small and consistent dimensions achievable with the process of theinvention, underdrain structures can support the sand filter mediumdirectly on the underdrain plates, thus eliminating the space normallyrequired for progressively smaller gravel layers positioned over theunderdrain to support the sand above, as in previous implementations.

[0010] A further benefit of the stamped-disc orifices is that the raiseddiscs during backflushing through the underdrain create an outwardlydiverging water flow pattern beneficial in clearing trapped particlesfrom the filter medium in the vicinity of the underdrain apertures.

[0011] Tests were performed on underdrain sections formed according tothe invention were tested for pressure loss. The pressure loss was foundto be much lower than head loss through underdrain structures withlaser-cut slots of the same total area of openings. This is due to theconsistency of the openings and the lack of irregularity along the edgesof the slots which are encountered with laser-cut slots.

[0012] It is thus among the objects of the invention to improve thestructure, manufacturing procedure and cost of metal underdrains forgranular filter beds, through use of a stamping process that formsraised discs in the metal structure, defining slits arranged generallyin a circuit. These and other objects, advantages and features of theinvention will be apparent from the following description of a preferredembodiment, considered along with the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a perspective view showing a granular filtration systemincluding an underdrain structure of the type to which this inventionrelates.

[0014]FIG. 2 is a view showing a flat metal plate punched or stamped toform raised discs and defining of slit openings in the plate, inaccordance with the invention.

[0015]FIG. 3 is a perspective view showing a raised disc and resultingslit openings in greater detail.

[0016]FIG. 4 is a view similar to FIG. 2, showing a variation.

[0017]FIG. 5 is a perspective view showing an underdrain structureformed by bending a punched plate such as shown in FIG. 2.

[0018]FIGS. 6A to 6E are views showing other shapes which can beemployed in the punching of the discs to form the slit apertures.

[0019]FIG. 7 is a schematic elevation view showing a plate used over anexisting underdrain system to provide advantages of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0020] In the drawings, FIG. 1 shows in perspective major components ofa liquid filtration system 10 partially in accordance with prior art andalso in accordance with features of the invention to be described below.The filtration system is formed as part of a tank typically of concreteand having a floor 12. This is a system of the type described in U.S.Pat. No. 4,331,542, as well as some of the other patents referencedabove. The filtration system, as described above and also in the '542patent, has a bed of granular material 14 which may be topped with alayer of bituminous material 16. Supporting the granular bed 14 is aperforated surface 18 at the bottom of the tank, preferably a corrugatedsurface as shown and as described in the '542 patent. Such a surfacepreferably is formed of a series of perforated longitudinal ribs 20which form triangular or trapezoidal cross-sectional shapes when placedagainst the floor 12.

[0021] In a preferred embodiment, as shown particularly in FIG. 5, eachperforated longitudinal rib 20 preferably has a horizontal flange 22 atleft and right extremities, these flanges of adjacent longitudinal ribsbeing slightly spaced apart in the completed underdrain unit 24, thusdefining troughs of the corrugations which are partly constituted by thetank floor 12 itself. As explained in U.S. Pat. No. 4,331,542, thelongitudinal ribs define inner spaces or chambers 26 which collectliquid filtrate passing through the granular bed 14 and through themultiplicity of perforations 28 in the ribs. This filtrate travels toand collects in a gutter or gullet 30 of the tank bottom.

[0022] As noted above, granular filtration systems and underdrain unitsof this general type are ordinarily equipped with liquid backwashingsubsystems, and also pressurized air (or gas), sometimes called scouringair, which is delivered adjacent to the underdrain perforations and tothe backwashing liquid to aid in the backwashing of debris from thefilter bed. FIG. 5 shows a form of air scour subsystem wherein air isdistributed through a tubular structural member 30 which has airdistribution outlet holes or slits shown at 32. This tubular structuralmember 30 also serves to retain the underdrain structures 20 downagainst the tank floor 35, thus serving a dual purpose, as disclosed inthe above referenced application Ser. No. 09/170,870, U.S. Pat. No.6,090,284.

[0023]FIG. 2 shows in flat plan view a sheet of metal 38, preferablystainless steel in a thickness of about 1.6 mm or slightly under{fraction (1/16)}″, from which the perforations or apertures 28 areformed in accordance with the invention. FIGS. 2 and 3 show that in oneembodiment, the apertures are formed by a stamping process, as by apunch press which stamps or punches out raised discs 40 from the flatsheet of material 38. The raised discs 40 formed in accordance with thisembodiment are generally circular, with diameters which may beapproximately ⅜″ to 1″. The discs are punched to a controlled depthwhich opens slits 42 between the disc and the surrounding flat field ofmetal, and these slits are highly consistent in width. A preferred widthfor this embodiment is about 0.010″.

[0024] The discs 40 are stably and rigidly retained on the flat metalsheet 38 by metal bridges 44 which are left remaining in the stampingprocess. The stamping die is shaped so as to leave these metal bridges44 and is precisely formed to result in highly consistently-sized slits42 serving as perforations for passage of filtrate. With theconsistently formed openings in sizes of about 0.010″ as is preferredfor some applications, the granular material 14 can have sand pileddirectly on the underdrain structure formed from the sheet metal plate38, and an intermediate bed of gravel in gradation of sizes can beeliminated. This can save up to about one foot of space in some types offiltration systems.

[0025] The metal stamping, a portion of which is shown in FIG. 2, isformed into an underdrain structure which preferably is trapezoidallyshaped as shown in FIG. 5. The bending of the stamped metal, along bendlines 46 and 48, is done subsequent to the stamping of the apertures, ina preferred implementation of the method, since the stampings are moreeasily accomplished on a conventional press when the metal piece is incompletely flat form. However, a special stamping die could be providedto punch the discs after bending of the plate.

[0026]FIGS. 2 and 3 show stampings which form raised discs 40 connectedby two opposed metal bridges 44. However, as discussed above, thesebridges can be greater in number, such as three or four around a circuitof slits, so as to assure a more secure, rigid and stable connectionbetween the raised discs and the surrounding flat metal area. Thus, inanother preferred embodiment three equally spaced bridges are leftremaining from the stamping process, thus providing three arcuate slitswhich are spaced apart 120° and which are separated by the bridges. FIG.4 shows an embodiment wherein raised discs 40 are retained to thesurrounding metal plate 38 with four such metal bridges 44. Thus, fourarcuate slits or punched slots 42 are formed as filtrate apertures,separated by the bridges 44. However, three such bridges are generallysufficient.

[0027] Although FIGS. 2, 3 and 4 show aperture slits 42 in a circularpattern, with circular raised discs 40 formed in the stamping process,the circuit of slits and the shape of the disc need not be circular. Itis sufficient that the slits be arranged in some form of circuit, theslits being separated by bridges, such that a raised disc of anypractical shape is formed in the punching or stamping operation. Acircle is an efficient shape, but FIGS. 6A-6E shows other shapes, asexamples, which can be employed. FIG. 6A shows schematically a series ofsix slits 42 a in a hexagonal pattern, with bridges 44 a betweenadjacent slits. Thus, a hexagonal raised disc 40 a is formed. FIG. 6Bshows a square raised disc 40 b, with a pattern of four slits 42 b in asquare arrangement, and this could be a non-square, rectangulararrangement.

[0028]FIGS. 6C, 6D and 6E show other shapes, including a triangularraised disc 40 c, with three slits 42 c in a triangular pattern; anoctagonal arrangement in FIG. 6D, with an octagonal raised disc 40 d;and an oval or elliptical shape in FIG. 6E, with a pair of slitapertures 42 e defined between the surrounding flat metal and a raisedoval disc 40 e. Only two slits 42 e are shown at FIG. 6E, but therecould be three or four slits.

[0029]FIG. 7 shows schematically another application for the punchedslot openings formed in accordance with the invention. The drawing showsexisting underdrain structures 20 a as implemented prior to theinvention, and these typically have {fraction (3/16)}″ diameter holesfor the filtrate. As noted above, these require a support gravel systemabove the underdrain structure, for supporting the fine granule filtermedium such as sand. These existing underdrain systems can be fittedwith a flat plate 52 placed across the tops of and supported by the flattop portions 20 b of the existing underdrain plates or structures 20 a.This eliminates the need for a support gravel bed above the underdrainstructures, increasing the vertical height available for the actualfilter medium by about 12″, and thus improving the filtering capabilityof the existing filter.

[0030] The plate 52 has punched openings as described above (not shownin FIG. 7). By leaving the existing underdrain structure 20 a in place,the illustrated arrangement makes this filter improvement veryinexpensive. The flat plates 52 can be attached to side walls 54 of thefilter system via an angled bend 55 in the plate, with stainless steelanchors 56 as shown, and attached to the tops of the existing underdrainstructures 20 a with stainless steel machine screws indicated at 58.Other methods of securing can be used. Filter sand or anthracite can beplaced directly on top of the flat plates 52 with their very fine,preferably 0.010″ punched openings. Because the plates 52 lie flat, theyneed to be somewhat thicker than the folded plates forming theunderdrain structures 20 described above, in order to support the filtermedium and withstand the forces generated by a high rate backwashwithout permanent deforming.

[0031] The above described preferred embodiments are intended toillustrate the principles of the invention, but not to limit its scope.Other embodiments and variations to this preferred embodiment will beapparent to those skilled in the art and may be made without departingfrom the spirit and scope of the invention as defined in the followingclaims.

I claim:
 1. In an underdrain filtration system which includes metalunderdrains comprising a series of elongated perforated metal ribstructures fixed against a floor and formed by bending so as to extendupwardly from the floor to define a filtrate plenum between theelongated rib and the floor, into which filtered liquid flows afterpassing through a bed of filtration media and through the perforationsin the elongated rib structure, the improvement comprising: a series ofstampings in the metal rib structure forming the perforations, whichcomprise slits positioned in arrays so that a plurality of such slitsgenerally form a circuit, such that at each array a raised disc extendsout from a surrounding flat area of the underdrain rib structure, theraised disc being integrally connected to the surrounding flat area bymetal bridges positioned between ends of adjacent slits and which areleft remaining from the stamping of the discs, and the slits definingsaid perforations in consistent width dimensions from slit to slit andfrom array to array, whereby consistency in the perforation size assuresproper and consistent pressure drop through the underdrain and properbalance of liquid flow during backflushing of the underdrain, andwhereby the raised discs cause a beneficial diverging pattern ofbackwash liquid flowing out of the underdrain and through the filtermedia when the underdrain is backflushed.
 2. The underdrain filtrationsystem of claim 1, wherein the discs are connected to the surroundingflat area by two to four spaced apart such metal bridges.
 3. Theunderdrain filtration system of claim 1, wherein the slits each have awidth of 0.010″, plus or minus 0.001″.
 4. The underdrain filtrationsystem of claim 1, in combination with sand as a filtration medium, thesand being positioned directly against the exterior of the elongated ribstructures of the underdrain system, the perforation slits being smallerthan the sand particles.
 5. The underdrain filtration system of claim 1,wherein the raised discs are essentially circular in shape.
 6. Theunderdrain filtration system of claim 5, wherein there are two to foursaid slits forming each array.
 7. The underdrain filtration system ofclaim 6, wherein the raised discs are about ⅜ to 1 inch in diameter. 8.A method for forming a filter structure for use in supporting a bed offine granular filtration medium, the method comprising: providing a flatsheet of metal, and stamping the flat sheet of metal in a punch presswhich has a die which will punch out a series of raised discs from thesurrounding flat area of the sheet of metal, such that at least twoslits are formed as apertures between each raised disc and thesurrounding flat area of metal, each disc being integrally connected tothe surrounding flat area by metal bridges positioned between ends ofadjacent slits and which are left remaining from the stamping of thediscs, and the slits being of a controlled and predetermined width forsupporting the fine granular filtration medium of preselected size. 9.The method of claim 8, in combination with the further steps of placingthe flat sheet of metal stamped with the discs and slits in anunderdrain filter system at a position supported above a floor to definea filtrate plenum between the sheet of metal and the floor, into whichfiltered liquid flows after passing through the slits, and placing thebed of fine granular filtration medium directly on the sheet of metal sothat a liquid material can be passed down through the granularfiltration medium to be filtered, with filtrate liquid flowing downthrough the slits in the plate.
 10. The method of claim 8, incombination with the further steps of bending the punched flat sheet ofmetal along a plurality of lines to form an elongated perforated metalrib structure to serve as an underdrain structure for an underdrainfilter system.
 11. The method of claim 10, further including placing anarray of such elongated perforated metal rib structures against a floorin an underdrain filtration system, with the rib structure extendingupwardly from the floor to define within each rib structure a filtrateplenum between the rib structure and the floor, and placing directly onthe array of elongated rib structures a bed of fine granular filtrationmedium such that the elongated rib structures support the filtrationmedium, such that filtered liquid passing through the granularfiltration medium is filtered by the medium to form a filtrate whichpasses down through the slits in the elongated perforated ribstructures.